Preparation

Cut 1 head of garlic crosswise in half. Place in large pot with ribs, 3 quarts water, and quartered onion. Bring to boil over high heat. Reduce heat to medium and simmer uncovered until meat is cooked through, about 50 minutes. Strain, reserving 6 cups meat broth (discard onions and garlic). Reserve ribs.

Preheat oven to 400°F. Wrap remaining head of garlic tightly in foil and place directly on oven rack. Place tomatoes and sliced onion on rimmed baking sheet. Drizzle with 1 tablespoon oil. Sprinkle with salt and pepper. Roast garlic, tomatoes, and onion until tender, about 1 hour. Transfer tomatoes and onion to processor. Using fingers, squeeze garlic from papery skin into processor. Blend until smooth. Set tomato sauce aside.